Electric generators, particularly portable engine-generator units, are often operated under variable load conditions. Such portable units are also often left unattended, which further causes them to be operated in sub-optimal conditions. It is well known that operating engine-generators under fluctuating load conditions can present a number of problems. As explained in U.S. Pat. No. 3,530,300 to Gunther, et al., diesel-fueled engine-generators are especially susceptible to problems associated with light-load conditions. In particular, a lightly loaded, diesel-fueled engine-generator may cause the engine portion of the machine to operate such that fuel within the engine remains unburned. This unburned fuel forms tar or carbon deposits, or both, which can collect within various parts of the engine, including exhaust pipes. This condition has historically been referred to as “wet stacking.”
Diesel engines also create soot during their combustion process. When an engine is operating under lightly load conditions, it tends to create more soot. Historically, the soot was exhausted from the engine to the atmosphere via exhaust pipes. More recently, however, increasing concern about, and regulation of particulate emissions has lead to changes in the manner in which diesel engines may legally operate. Modern diesel engines, including so-called Tier IV Interim and newer engines, are equipped with particulate filters that serve to capture and remove soot from engine exhaust. The problem is that these particulate filters, which are also called diesel engine particulate filters or DPF, tend to clog with the particulate that they filter. Those clogs inhibit engine exhaust, which may significantly or entirely decrease the operability of an engine. To combat this DPF-clogging issue, modern diesel engines are often equipped with a “regeneration” feature. Regeneration involves a process in which the engine doses a DPF with diesel and then ignites that diesel to burn-off accumulated soot. The soot is reduced to ash, which falls away from the DPF, thus clearing the DPF and the engine exhaust system.
Complicating DPF regeneration in engine-generator applications is the effect of generator loading on engine performance. Engine exhaust temperature is proportional to generator loading. Thus, a lightly loaded generator causes the engine to operate with lower exhaust temperature than a generator operating under greater loading conditions. The problem is that regeneration efficiency is inversely proportional to exhaust temperature. That is, regeneration works better when an engine is operating with higher exhaust temperatures. So, in engine-generator applications, regeneration is more efficient when a generator is operating under loaded conditions.
It is known in the art that wet stacking may be mitigated by connecting to the generator a “dummy” load in parallel with the actual, or “real,” load. Such dummy loads often comprise banks of resistors that may be switched in steps, often automatically, in inverse proportion to the real load. In this way, a generator may be connected so that it always experiences some minimum demand. As Gunther, et al. explains, this is represented by the equation PG=PRL+PDL, where PG is the total generator demand, PRL is the demand of the real load, and PDL is the demand of the dummy load. This same method of operation helps reduce soot in the engine.
But despite the benefits of the system of Gunther, et al., and other similar configurations, such schemes tend to be wasteful because they cause the engine portion of the engine-generator to burn a set amount of fuel, at all times, in order to maintain PG. This is because the engine must burn more fuel to maintain an increased generator output. So while the system is burning cleaner, it uses more fuel than necessary. In a world of rapidly decreasing oil supplies, and rapidly increasing fuel costs, it is desirable to balance the benefits of minimum generator loading with economic realities. Said differently, it is advantageous to decrease soot production and wet-stacking events with dummy loads (aka load banks), while at the same time being mindful of the fuel costs associated with operating dummy loads. Furthermore, load banks have not previously been employed to aid with DPF regeneration.